1. Field of the Invention
The present invention generally relates to a magnetizing method for a permanent-magnet motor, and in particular to a method of magnetizing a permanent-magnet material used in a rotor structure of a permanent-magnet motor having reversed salient-polarity to make effective use of a reluctance torque together with a magnetic torque.
2. Description of the Prior Art
Conventionally, permanent-magnet motors are used as examples of variable velocity motors in many cases. A permanent-magnet motor has a rotor which includes plural pieces of permanent magnet. In such a permanent-magnet type motor, a permanent magnet is formed by, e.g., molding a solid mass of magnetic powder material. Therefore, the molded permanent magnet material has no magnetic polarity at an initial stage of manufacturing process of a motor. Namely, by effecting a magnetizing process of a permanent magnet material, the magnetic polarity is given to the permanent magnet material for the first time to form plural pieces of permanent magnet members.
When a magnetizing operation of a permanent magnet material is carried out, a center axis of the resultant permanent magnet member must be positioned in a normalized relationship in accordance with an axis of a magneto-motive force generated by a magnetizing current flowing through windings located on a stator so that the magnetic pole is fixed to a reference or normalized position without any displacement. This alignment of the magnet axis and the magneto-motive force axis was conventionally performed by a mechanical construction.
If magnetization of the permanent magnet material is carried out with a displacement between the magnet axis and the magneto-motive force axis, the resultant magnetization is insufficient in quantity, resulting in a reduction of a driving efficiency of a motor and causing an error in detecting a rotational position of a rotor. Thus, the alignment of the magnet axis and the magneto-motive force axis is essentially necessary.
In a conventional magnetizing method for a rotor having an outward salient polarity as shown in FIG. 13, electric current for alignment is applied from an alignment current source to flow through windings on a stator to generate a magneto-motive force. In the meanwhile, the rotor core 103 includes four pieces of permanent magnet 106 for four polarized sections and joint members 108 of electro-magnetic steel for coupling the adjoining permanent magnet members to each other. In this construction, when the alignment current is flown through the windings, the magneto-motive force generated by the alignment current acts as an absorbing force for absorbing the joint members. Thus, the rotor is rotated toward the normalized position for establishing the alignment by the absorbing action due to the magneto-motive force generated by the alignment current. By this alignment operation, the magnet center axis is set to a specified relationship of e.g. right angles with respect to the axis of the magneto-motive force generated by the alignment current.
Then, under the condition that the rotor is set in the normalized position, the magnetization is carried out by applying a magnetizing current from a magnetizing current source to flow through the windings. In this magnetizing operation, the magnet center axis can be coincident with the axis of the magneto-motive force generated by flowing the magnetizing current through the windings so that the rotor is maintained in the normalized position.
In order to effectively take advantage of a magnetic torque as well as a reluctance torque, another conventional method of magnetizing a permanent magnet material was developed for fabricating a rotor having a reversed (i.e., inward) salient polarity as shown in FIGS. 9 and 10. In this construction, the material of the permanent magnet is firstly embedded in a rotor body of a motor and then magnetized. It is noted here that the reluctance torque is a component of the total torque when the motor is operating synchronously. It results from the saliency of the poles and is a manifestation of the poles attempting to align themselves with the air-gap magnetic field.
FIGS. 9 and 10 show an example of a conventional magnetizing method for a permanent-magnet type motor to have a reversed salient-polarity construction effectively taking advantage of a reluctance torque along with a magnetic torque.
In this construction, plural permanent magnet material portions 52a and 52b are embedded for each pole section inside a rotor core 51 of a rotor 50. The rotor core 51 is essentially composed of high permeable materials such as, e.g., iron or formed by laminating electro-magnetic steel plates. Then, the embedded permanent magnet materials 52a and 52b are magnetized by applying a magnetizing current from a magnetizing current source (not shown) flowing through windings 21 provided on a stator 20.
The rotor 50 is rotated on a rotating shaft 54, generating a magnetic torque and reluctance torque due to a rotational magnetic field generated by the current flowing through the windings 21 on the stator 20. Thus, the magnetization of the permanent magnet materials 52a and 52b embedded in the rotor 50 is carried out by flowing electric current between, e.g., R-phase and S-phase of the three phases through the windings 21 while the rotor 50 is fixed to a normalized position specified for accurate and complete magnetization as shown in FIGS. 9 and 10.
In this magnetizing operation, however, if the position of the rotor is even only slightly displaced from the normalized position, the reluctance torque acts as a rotating force to rotate the rotor 50, which undesirably causes a shift in position of the rotor to result in insufficient magnetization.
In order to avoid this problem, the rotor 50 is incorporated in a motor 1 as shown in FIG. 11, with its shaft end 53 being securely regulated in position by providing a securing jig member 55 for preventing the rotor from rotating.
Alternatively, as shown in FIG. 12, the rotor 50 is incorporated in a cylindrical magnetizing yoke 4 with the shaft end 53 fixed in position to a securing jigmember 56 for preventing rotation of the rotor.
However, in the mechanical alignment of the magnet axis and the magneto-motive force axis in these conventional magnetizing methods mentioned above, as the magnetizing current is required to have a large value of several tens or several hundreds times larger than the rated current value, therefore the rotational force of the rotor 50 by the reluctance torque is very strong. Accordingly, there has been a problem that the securing jig member for preventing rotation of the rotor is undesirably damaged, or a mechanism for transferring a driving force from the rotating shaft is damaged in some cases.